Originally Posted by
whitis
Seems you are trying to solve problems created by a flawed solution to a previous problem. You want to apply the drawbar force from spindle to drawbar, not from housing (or quill) to drawbar. This doesn't put any force on the spindle bearings (or push on the quill) but does put force on the less critical drawbar bearings. It does, possibly, involve an extra set of thrust bearings because the spindle shaft rotates (but see below). Also, the methods you suggest still put clamping force on the precision spindle bearings - which could be a real problem if it wasn't for the fact that DIY power drawbars don't appear to provide adequate clamping force.
One issue i see with most of the low end power drawbars/ATCs is the clamping force. It appears that professional machines use something like 1 to 4 tons of force. The old fashioned manual drawbar, i.e. a big bolt, can have huge clamping forces. At 100PSI, that is a lot of square inches. I was looking at an air operated vise in a tool catalog, the other day - it looked like its piston was something like 6" diameter. Sure for the smaller machines, you are looking at smaller tools and lower cutting forces and lower speeds (thus less imbalance) so you may not need two tons of clamping force but you still may need a surprising amount - even with manual drawbars, slipping can be an issue on smaller machines. And even the big machines can have trouble with clamping. Grease gets on the toolholders and the spindle taper. So, you may be looking at a serious size pneumatic piston, hydraullics, or a screw mechanism.
Belleville washers gain you nothing in terms of clamping force. Whatever force they exert, you have to exert to release them. And they may come at a potential mechanical price in that you now have to change the direction of applied force in a way that may be less convenient and your length of travel may be limited. One of the things the washers do for you is insure that the system fails in the tool retained mode - so that you don't drop the tool due to a loss of pressure. Though failure to drop the tool before picking up the next might be worse. Another thing the washers do for you is that they make the load on the drawbar bearings occur when the spindle is not rotating. They also may eliminate the need to apply force in more than one direction, though they make it hard to separately control the retention and tool eject forces.
Another possibility with the belleville washers, is to eliminate the drawbar bearings entirely by having the clamping mechanism withdraw far enough that it does not engage either the spindle or the drawbar while the system is not spinning. Overall, it isn't too hard to do it right. You either pinch from the top of the drawbar to an anular disk at the top of the spindle or if that interferes with spindle assembly, you pinch to a disk at the bottom of the spindle, near the nose.
Of course, working with a spindle that wasn't designed to be used for ATC, complicates matters. Enough that it may be easier to replace the spindle. The quill fed machines are more awkward, mechanically.
How much clamping force does one need? Not that easy to answer and depends a lot on the machine, application, etc.
- Needs to exceed any downward forces on the tool encountered during milling.
- Needs to exceed the rotary cutting forces on the tool, scaled by various
mechanical factors, as these are
- If your spindle/toolholders aren't keyed, the force needs to be high enough to prevent slippage in the taper due to the torque required for cutting.
- Needs to exceed the forces of imbalanced tools, tool holders, scaled appropriately. Boring heads, for example, can be hideously imbalanced. Flycutters can also be very unbalanced. Centrifugal forces are proportional to the square of the speed. Some of these forces have a significant mechanical advantage over the drawbar.
The deep tapers used on tool changers are not very good at holding compared to shallow tapers used for collet systems.
On a little sherline spindle with a 3/8-16 draw bar, if you apply 5 pounds of force to a 6" wrench, you are probably providing around 500lbs of clamping force - on a small tool in a shallow taper (i just measured 10pounds on a shorter wrench before the battery in my fish scale died). The shallow taper multiplies that force many times over. Joe Vicar's ATC uses a 1" air cylinder, which would provide about 78lbs at 100psi, and this on a deep taper. Don't think I will try that on a boring bar, fly cutter, milling in a deep pocket, or machining 316. The Hoss ATC for tormach uses a slightly larger cylinder that is capable of providing 176lbs at 100psi.
Tormach doesn't mention the normal tightening force but says "Only a few thousandths of an inch of stretch is needed to generate hundreds of pounds of force by a steel drawbar". They also say their tool holders slip resulting in a Z error of 10mils on heavy cuts - apparently even when dry.